Upon activating a dispenser crating a PrimedTnt entity, the entity is created 1/2 a block too high up, and is partially inside the block above. Occasionally, this causes the entity to snap on top of that block, but it usually falls out of the bottom of the block afterwards before exploding. While the entity is within the block, it can be moved further upwards using explosions.
The main situation this causes problems is while using TNT cannons, which rely on containing the TNT immediately in front of the dispensers using a block above them, which allows for continuous fire. Instead of the TNT remaining in the 'Propellant chamber', it can become launched upwards through the block above where it was spawned, as the bug caused it to be spawned partially inside said block, causing the TNT cannon to fail.
All images were taken almost immediately after the PrimedTnt entity was spawned by the dispenser. Occasionally, the TNT moved upwards slightly before the screenshot was taken, resulting in less than 1/2 of the entity being visible, and the rest of it being black.